The bovine leukemia virus (BLV) belongs to a unique class of retroviruses like HTLV-I, II or III in which productive infection is highly restricted both in vivo and in vitro. To examine the molecular basis for the control of BLV expression, we excised the LTRs from cloned proviruses and fused them to be bacterial chloramphenicol acetyltransferase (CAT) gene. Plasmids carrying the CAT gene controlled by the entire BLV LTR or partially deleted LTRs were introduced into a variety of cells. The BLV LTR was inactive as a promoter of transcription in all cell lines tested except those previously established as BLV producers (e.g., FLK-BLV cells). Transcriptional regulatory sequences in the LTR were first identified by deletion mapping. We found that levels of CAT activity in FLK-BLV cells were reduced by 90% when LTR sequences located 100 bp to 170 bp upstream of the CAP site were deleted. Furthermore, removal of LTR sequences downstream of the RNA start site reduced CAT expression by 87%. In further experiments, a 75 bp LTR subfragment encompassing the region 100 bp to 170 bp 5' of the RNA CAP site was cloned into pSV E cat (a pSV2 derivative lacking the 72 bp repeats and requiring "enhancers" for efficient CAT expression). Insertion of the BLV 75 bp fragment into pSVE cat resulted in high level CAT expression only in BLV producer cells. This specific enhancement was orientation but not position independent. A 250 bp fragment containing the long R region of the LTR also activated CAT expression from the heterologous promoter, but only when located immediately 3' of the RNA start site. This activation was not cell-specific and was relatively orientation independent. Thus, the BLV LTR appears to possess two independent elements regulating gene expression. Additionally, experiments have been initiated to understand the mode of gene regulation and antigenic drift of the equine infectious anemia virus (EIAV). Full-length clones of EIAV have been hybridized with HTLV-III and found to be genetically related. Furthermore, DNA sequence analysis shows that the relatedness of EIAV and HTLV-II is substantial, especially in the pol region of EIAV.